The present invention relates to a method and apparatus for controlling the operation of a printing machine, and more particularly to improvements in a method and apparatus for controlling or regulating the ink feed and other parameters affecting the density of colors which are applied to various zones of sheets during each run of the press. Still more particularly, the invention relates to improvements in a method and apparatus for monitoring and evaluating the quality of color reproduction in multi-color printing in order to ascertain the need for adjustment of the color-affecting controls on the press.
It is well known to provide each sheet of the material being imprinted with a so-called color control strip which consists of a row of aligned patches or blocks in each color. It is also known to optically scan the strips and to generate electric signals which are thereupon utilized to ascertain the density at the center of each patch in a strip. For example, commonly owned pending patent application Ser. No. 112,155 of Wirz et al. discloses an apparatus which can be used to generate signals denoting the characteristics of minute increments of patches in a strip and to thereupon evaluate such signals with a view to ascertain the density of the central region of each patch. A sensing pickup head is moved lengthwise of the strip and carries suitable components which generate signals denoting the intensity or density values of successive increments of consecutive patches in a strip. The signals are stored according to the colors of the respective patches, and the evaluation of such signals renders it possible to determine the boundaries between neighboring patches on the basis of detection of more or less abrupt changes of color density when the sensing head moves beyond a preceding patch and begins to scan the next-following patch of a strip. Once the apparatus ascertains the boundary between two neighboring patches of a strip, it can calculate the center of the next-following or preceding patch and hence the density in the region of such center. Determination of densities in the central zones of successive patches is desirable and advantageous because this is much less likely to furnish distorted and hence misleading readings than if the measurements were taken close to the boundaries between neighboring patches. The misleading readings would be translated into improper adjustments of color-affecting controls in the press.
The apparatus of the copending application Ser. No. 112,155 operates quite satisfactorily. However, it can happen that, owing to improper printing of strips on the sheets or for other reasons, the patches are not applied with requisite clarity or accuracy, the neighboring patches overlap each other and/or the neighboring patches are separated from each other by seams having the color of the bare stock, i.e., by gaps wherein the sheet exhibits the basic white color. This can cause problems in connection with the determination of boundaries between neighboring patches of the strips.